1. Field of the Invention
This invention relates to an adhesive coating composition which can be heat activated without becoming C-staged, and later can be C-staged. More particularly, this invention relates to an adhesive coating composition which: (1) is non-blocking; (2) can be heat activated without becoming C-staged; (3) later can be C-staged; and (4) is suitable for bonding a wire to a surface, to another wire, or to itself.
2. Description of Prior Art
Heat activatable adhesive compositions can be applied to a substrate as a thin, non-tacky film, can be heat activated and when activated, become and remain tacky for a period of time after activation. Most heat activated thermoplastic adhesives resolidify rapidly upon cooling and have a short open time, i.e., after activation of the adhesive, the adhesive remains aggressively tacky and bondable below the activation temperature for only a short time. Bonding operations must be carried out quickly.
In many bonding operations, it is necessary to employ adhesive compositions which result in adhesive bonds exhibiting resistance to elevated temperatures. It is known that improved temperature resistance of adhesive compositions can be achieved by the incorporation of crosslinking into the compositions. Particular crosslinking agents known in the art include the polyepoxides and optional epoxy polymerization catalysts described in U.S. Pat. No. 3,723,568 to Hoeschele wherein crosslinking is achieved by reactions with available sites in the base polymers.
U.S. Pat. No. 4,137,364 to Ball et al. describes crosslinking of an ethylene/vinyl acetate/vinyl alcohol terpolymer using isophthaloyol biscaprolactam or vinyl triethoxy silane. Crosslinking here is achieved before heat activation, but a higher level of performance is attained by additional crosslinking induced by a heat cure after application of the adhesive. Another example of thermal crosslinking is the polyamino bis maleimide class of flexible polyimides described in U.S. Pat. No. 4,116,937 to Jones et al. These compositions can be hot melt extruded up to 300.degree. F. and undergo crosslinking at temperatures above 300.degree. F. In both of these examples of thermal crosslinking, crosslinking is achieved by reactions of the particular crosslinking agent with available sites of the base polymers.
Thermosetting adhesive compositions are known which combine polyisocyanates with binders which contain groups that are reactive toward isocyanate groups. However, considerable difficulties result from the fact that the isocyanate group reacts with the envisaged reactant, e.g., a polyol or a polyamine, at as low as room temperature. As a result, it is extremely difficult to formulate a combination having a long shelf life.
In order to overcome this difficulty, two-component systems are frequently used. The reactants are stored in separate formulations, which are mixed only just before application; the reaction then takes place spontaneously or is accelerated by heat and/or a catalyst.
For example, such a two component system is described in U.S. Pat. No. 4,029,626. Disadvantages of this system are the necessity for separate formulation and storage of the two components, the difficulties in precise metering and thorough mixing before application, and the danger of blockage of the mixing apparatus and applicator as a result of premature reaction.
Another conventional method of preparing polyisocyanate materials having a long shelf life starts from non-crosslinked prepolymers which contain only a small amount of free isocyanate, atmospheric moisture serving as the crosslinking agent. This method is described in, for example, French Pat. No. 1,237,936. The disadvantages in this case is that the hardening, which begins at the surface, extends only very slowly to the deeper regions of the coating, so that the final properties are acquired only after weeks or months. No hardening at all occurs between large areas or in cavities.
Stable systems also can be formulated if the polyisocyanate is first reacted with a monofunctional reactant. The resulting adduct is referred to as a blocked isocyanate if it is less heat-stable than the product of the crosslinking reaction to be carried out subsequently. Under the action of heat, the blocking agent is eliminated, and the polyisocyanate undergoes crosslinking, forming more heat-stable bonds.
The principle is described in, for example, Frisch, "Fundamental Chemistry and Catalysis of Polyurethanes", Polyurethane Technology, Bruins, editor (Interscience Publishers, New York, 1969), pp. 11-13; and U.S. Pat. Nos. 4,068,086 and 4,251,428.
Because of the eliminated blocking agent, such formulations have technical and economic disadvantages: if the blocking agent remains in the crosslinked material, it changes the properties of the latter; if it vaporizes, it either has to be recovered or has to be removed from waste air.
U.S. Pat. Nos. 3,475,200; 4,251,427; and 4,251,428 describe polyisocyanates which melt at above 100.degree. C. and at above 130.degree. C., respectively, and which, with special polyols, give combinations which have long shelf lives and which can be hardened by heating to give coatings for fabrics or woven hoses.
The industrial disadvantages of the combinations described in these publications is that products having a long shelf life can be obtained only with a very small number of polyols having a special composition; therefore, these combinations are suitable for a limited number of uses.
U.S. Pat. No. 4,093,675 to Schure et al. describes an adhesion composition containing a poly(ester/ether) block copolymer (I) with terminal OH, which is chain extended and carboxylated with a dianhydride, to form a carboxylated poly(ester/ether), block copolymer having the formula: EQU [O------OOC--R(COOH).sub.2 --CO--).sub.n ]
wherein --O------O-- is a poly(ester/ether) block copolymer; R is alkyl, cycloalkyl or aryl; and n is 1 to 2. The thermoplastic block copolymer may be hardened to a thermosetting copolymer which contains a cross-linked component as well as a linear, flexible copolymer component. The crosslinked copolymer is a block copolymer in which one of the blocks itself is a block copolymer, and can resist creep at 150.degree. C.
U.S. Pat. No. 4,430,479 to Merton et al describes an adhesive composition which comprises a thermoplastic polymer (I); 1-100 parts by weight of a crosslinkable polymer (II) being a hydroxy-substituted polyester of polyacrylate, or a polyamine; and a crosslinking agent, preferably a di- or polvisocyanate. (I) is preferably a polyester, polyurethane, polyamide, rubber or ethylene/vinyl acetate polvmer, and optionally contains up to 100 parts by weight per 100 parts by weight of a conventional tackifying resin (typically hydrogenated and dehydrogenated rosin esters). The compositions have good adhesive strength; they exhibit delayed tack on heat activation, which is achieved at relatively low temperature and show high temperature resistance. They are non-blocking and may be applied to substrates well in advance (about 60 days) of the actual bonding operation.
U.S. Pat. Nos. 4,151,319 and 3,578,622 to McGrath concern a heat-activated adhesive which comprises (a) an acrylate polymer capable of room temperature, pressure-sensitive adhesion of at least 100 g/cm width; and (b) a room temperature-solid tackifying resin that provides increased room temperature tackiness when added to component (a), or to natural rubber, but which is in an amount sufficient to reduce room temperature, pressure-sensitive preadhesion of the adhesive to less than the pressure-sensitive adhesion of component (a) prior to addition of component (b). The tackifying resin permits conventional and effective room temperature handling of the coated sheets, while leaving the adhesive with the capacity for strong heat activated bonds.
U.S Pat. No. 4,194,618 to Norton describes a coated abrasive article comprises a backing member having abrasive grain adhered to it by an adhesive bond. The article has two of its ends joined together by a layer of an adhesive composition, which in its initial state is a preformed adhesive film which has been partially cured but which can still be activated at a temperature suitable for splicing the ends together. The adhesive film is the reaction product of an adhesive composition comprising a mixture of (a) a hydroxy-terminated polyurethane-polyester of hydroxyl No. 2-15; (b) an active hydrogen component having an active hydrogen equivalent weight of 27 to 500; (c) a component having free available isocyanate groups; and (d) a chlorinated aliphatic hydrocarbon, chlorinated aromatic hydrocarbon and/or alklylated polystyrene, this component being miscible with components (a), (b) and (c).
U.S. Pat. No. 4,476,259 to Kordomenos describes a solvent based thermosetting composition comprising: a hydroxy functional epoxy ester resin having a number average molecular weight (M.sub.n) between 1,000 and about 5,000; and (b) a polyfunctional, hydroxy-reactive, crosslinking agent. The epoxy ester resin is formed by the reaction of diepoxide, which has been chain extended with diphenol and dicarboxylic acid, with hydroxyfunctional, secondary amine in a chain terminating reaction, in about a 1:1 equivalent ratio. The crosslinking agent is an aminoplast crosslinker or a blocked isocyanate crosslinking agent comprising isocyanate groups blocked by reaction with an active hydrogen bearing blocking agent.
U.S. Pat. Nos. 3,646,572 and 4,097,684 to Burr describe a catalytic adhesive for wire scribed circuit boards which comprises a flexible adhesive resin, alone or in combination with thermosetting resins. The flexible adhesive resins include epoxy resins, polyvinyl acetal resins, polyvinyl alcohol, polyvinyl acetate, natural and synthetic rubbers and acrylic polymers and copolymers. Preferred for use as the adhesive resin are natural and synthetic rubbers such as chlorinated rubber, butadiene-acrylonitrile copolymers, and acrylic polymers and copolymers.
The use of this adhesive in a process for manufacturing wire interconnection boards is mentioned in U.S. Pat. Nos. 3,674,602 to Keogh et al. and 3,674,914 to Burr. The adhesive layer is in the form of a partially cured thermosetting resin which is non-tacking at room temperature, but which, upon application of heat below the thermosetting temperature, becomes malleable and provides an adhesive bond when heated momentarily and cooled. In a step of the aforementioned process, after the wire conductor pattern is completed, it is permanently bonded to the substrate by pressing the wire conductors into the adhesive layer with heat and pressure, further curing the adhesive, or, by covering the wire conductors with a glass cloth reinforced, epoxy prepreg and then laminating the wire conductors into the adhesive with heat and pressure. Adhesives based upon those described in the aforementioned Burr patents have been used commercially for many years in the manufacture of wire scribed circuit boards.
In the aforementioned laminating step, occasionally, wire conductors laid down in the correct position would "swim" away when a cover layer of epoxy impregnated fiberglass was laminated over the wire conductors under heat and pressure. The hydraulic pressure of the melted epoxy resin, and partially melted adhesive layer beneath moved the wire conductors. Attempts to eliminate the aforementioned wire swimming, by increasing the partial cure of the adhesive before wiring, or by baking after the wire conductor pattern had been completed, or by modifying the laminating conditions were not successful.
The volume resistivity of adhesive layers of the type described by Burr is about 1.times.10.sup.4 to 1.times.10.sup.6 Megohms because high rubber content is needed to give tack for ultrasonic or heat bonding of the wire to the adhesive In some applications requiring high insulation resistance, higher volume resistivity is preferred.
U.K. Patent Specification No. 1,504,252 by Nicolas suggests wire scribing with a wire coated with adhesive or with a layer which can be rendered adhesive preferably immediately before scribing, and further suggests a dry film which is made self-adhesive by heating or passing the film through a suitable solvent. There is no teaching by Nicolas of non-blocking, heat activatable, wire coating compositions which can be cured to a C-stage and which are resistant to the effects of solvents and of soldering temperatures.